1. Field of the Invention
The present invention relates to a powder constant-volume feeder and more particularly to a powder constant-volume feeder especially suitable for feeding powders of metals, ceramics or plastic polymers such as borides, silicides, intermetallic compounds, or complexes, as flame spraying materials, to a plasma spray gun continuously in a constant volume while controlling the bulk density to a constant level.
2. Prior Art
Heretofore, it has been extremely difficult to feed powders of metals or ceramics as spraying materials in a constant volume while controlling their bulk density to a constant level, because those powders are extremely fine, ranging in diameter from 1 to 200 .mu.m.
Generally, there is known a powder feeder of a structure provided with a powder storing hopper, in which powder is allowed to drop from the hopper. According to this conventional structure, however, the powder is deposited in the opening of the hopper, causing unevenness in the dropping of the powder from the hopper, or a change in the storage condition of the powder in the hopper causes constant variations in bulk density of the powder, so it has been difficult to feed powder in a constant volume.
For example, in "Powder Bulk Handling Technique" edited by Corporation Japan Powder Industrial Technology Association (1985) there are introduced a belt feeder, a chain feeder, a screw feeder, and a table feeder, as feeders capable of being utilized in constant-volume feed. In all these feeders, however, the powder bulk density varies according to changes in the amount of powder charged into the hopper, so there still has been a problem in constant-volume feed of powder. In other words, since the bulk density of powder charged into the hopper changes under the influence of the own weight of the powder, it has been difficult to maintain the powder density at a constant level.
There has also been introduced a powder feeder in which powder particles capable of being maintained at a constant weight even under changes in powder bulk density are detected continuously and the amount of powder to be fed is controlled in accordance with the results of the detection to thereby attain a constant-volume feed of powder. For example, reference is here made to FIG. 9 which is a schematic diagram of a belt scale type continuous constant-weight feeder. As shown in the figure, the weight of powder X discharged from a hopper 100 of this feeder is detected on a belt 101 and on the basis of the detected value there are adjusted the speed of the belt 101 and the degree of opening of an adjusting gate 102 for the outlet of the hopper 100 to thereby feed a constant weight of a powder X continuously.
However, such feeder based on the detection of weight involves the problem that the control timing for the powder already fed is delayed because the control is made after the detection. Moreover, the entire apparatus becomes complicated because of additional necessity for the provision of a detector and a controller.
Further, in Japanese Patent Laid Open No. 10324/1982 there is described an apparatus in which a canister having on the inner surface thereof a plurality of rake-up blades and a powder conveyor are combined and the powder in the canister is allowed to drop onto the conveyor by means of the rake-up blades to effect a constant-volume delivery of the powder. However, such conventional apparatus involves the problem that the powder bulk density is not constant, varying in dependence upon the amount of the powder charged into the canister. More particularly, the larger the amount of the powder charged into the canister, the higher the powder bulk density, and vice versa. Therefore, it is difficult to keep the bulk density constant. Further, because of the use of a screw feeder as the conveyor, there easily occurs clogging of the passage from the interior of the canister to the exterior thereof with the powder, or defective feed due to wear, thus causing an obstacle to a constant-volume feed of powder.
A powder blender/feeder unit used in a plasma sprayer is described in a catalog, "PRODUCT DATA SHEET" No. PF-500-2, published by Dresser Co., an American company. However, the structure of this unit is the same as that described in Japanese Patent Laid Open No. 10324/1982; that is, it is difficult to feed a constant volume of powder continuously while controlling its bulk density to a constant level.
Thus, in the conventional powder feeders for a constant-volume feed of powder, there is a problem in point of variations in the powder bulk density caused by changes in the amount of powder charged, and a problem is also found in the powder conveying path. These problems have been an obstacle to the attainment of a constant-volume feed of powder.